a foam core. I had issues with mine cracking on the mounting pillars. I repaired it with epoxy and I think it'll be much stronger than factory b/c the plastic was way too thin for the anchors. Its better than the early factory wings, but that particular area is a weak spot. Of course, depending upon how well the fiberglass is laid, the replacement may or may not be any better. You would likely know just by feeling it if its a quality part or not. Z1 sells the replacements, as do several places on ebay, but I have not personally inspected the fiberglass pieces. The mounting holes are a bit different than factory (not sure why) but since you have an NA hatch now, that should not be an issue.